The present invention relates to vehicle speed control, and more particularly to headway limiting in adaptive cruise control for automotive vehicles.
Automatic vehicle cruise control systems have been developed for maintaining the speed of a vehicle at a driver-selected speed. In conjunction with these known cruise systems, an adaptive cruise control system has been developed for detecting the presence of, and the distance to, a preceding vehicle and for adjusting the vehicle speed to maintain a following distance from the preceding vehicle when it is moving slower than the driver-selected speed. In essence, the source vehicle speed is controlled to the speed of the preceding vehicle with a speed dependent separation being maintained from the preceding vehicle, wherein the source vehicle speed is limited at a driver-selected speed.
Adaptive cruise control systems typically have conventional cruise control that may be overridden in certain circumstances by an adaptive vehicle speed control routine. The conventional cruise control, when activated, may include a control function designed to minimize a difference between the actual vehicle speed and a cruise-set speed, which is selected by the driver. The adaptive cruise control system adapts any active conventional control to the environment external to the controlled source vehicle by attempting to account for vehicles preceding the source vehicle within a speed dependent separation interval of the source vehicle and in the path of the source vehicle.
One form of adaptive control lies in the reduction of the cruise-set speed below the driver-selected speed by an amount determined and periodically updated to provide controlled following of sensed preceding vehicles traveling slower than the driver-selected speed. A speed command is generated, based in part, on the speed relationship between the source vehicle and the preceding vehicle. The cruise set speed is limited accordingly, to adapt the source vehicle speed to that of the preceding vehicle and provide a controlled following relationship.
Adaptive cruise control systems, such as the system described in U.S. Pat. No. 5,014,200, incorporated herein by reference and assigned to the assignee of the present invention, provides a speed dependent following distance that is adjustable by the driver. This adjustment affects a speed multiplier term used in the determination of the speed dependent following distance. For a fixed speed multiplier, the desired following distance should increase with increasing vehicle speed.
The present invention resides in an adaptive cruise control system that provides smooth adaptive cruise control following operation of a source vehicle by limiting the headway to keep it lower than a predetermined maximum. In the present invention, following distance limiting (headway limiting) allows the source vehicle to follow the preceding vehicle at the maximum distance or time interval that is predetermined to provide a smooth xe2x80x9cnon jerkyxe2x80x9d following operation. The system informs the driver of the source vehicle that the following distance and thus the amount of time or distance of headway is less than what the driver expected. The present invention accomplishes this objective through systematically limiting the headway. In particular, a time related speed dependent term used in calculating the desired headway is assigned prescribed limits, to maintain a headway that is less than a predetermined maximum headway that maintains the preceding source vehicle spacing within a preferred operational range for the adaptive cruise control system.
Limiting the headway accounts for the practical maximum range limitations of forward looking sensors and allows smooth adaptive cruise control following operation of the source vehicle, especially during high speed operation of the source and preceding vehicles without requiring the driver to adjust the speed dependent following distance. This operation is desirable due to the practical maximum range limitations of forward looking sensors used in adaptive cruise control systems, wherein the performance of the adaptive cruise control system can be less than optimal when the desired headway is near or beyond a maximum range for the system""s sensor.
When no vehicle is detected to be in the path of die source vehicle, the adaptive cruise control system maintains or accelerates the source vehicle to the driver-selected speed. Conventional adaptive cruise control systems provide a speed dependent headway when a preceding vehicle moving slower than the driver-selected speed is encountered in the path of the source vehicle. Therefore, it is possible that the source vehicle will accelerate and decelerate, as the distance to the preceding vehicle moves into and out of the maximum sensor range resulting in a rough, xe2x80x9cjerkyxe2x80x9d adaptive cruise condition. This occurs when the preceding vehicle is traveling slower than the driver-selected speed of the source vehicle but faster than a relatively high speed resulting in headway corresponding to or exceeding the maximum sensor range.
For a hypothetical example in a conventional adaptive cruise control system, with a driver-selected speed of 120 kph, a 100 meters maximum range system, and no preceding vehicle ahead, the source vehicle speed would be 120 kph. If the source vehicle approaches a preceding vehicle traveling at 110 kph, the source vehicle would be decelerated by the system when it approached within 100 meters of the preceding vehicle. This occurs to bring the source vehicle down to a speed of 110 kph. The actual distance between vehicles can be lower than the desired headway based on the driver""s setting, by the time the source vehicle decelerates to 110 kph. The source vehicle then decelerates slightly to meet the desired headway. When the deceleration causes the distance to exceed the maximum sensor range, no preceding vehicle will be perceived and the source vehicle will accelerate back toward 120 kph. When the preceding vehicle comes within 100 meters again, the source vehicle would decelerate. Based on the situation, this cycle can be repeated and the adaptive cruise control system can result in a xe2x80x9cjerkyxe2x80x9d cruise condition.
Therefore, the present invention limits the maximum headway when a source vehicle is operating in an adaptive cruise control system mode.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.